(i) Field of the Invention
The present invention relates to a polyurethane foam for cushioning materials.
(ii) Description of the Related Art
A cushioning material has heretofore been considered ideal when its stress-strain curve extends linearly like metal springs while retaining a suitable degree of rigidness. Conventional cushioning materials have therefore been developed with a view toward making their stress-strain curves closer to those of metal springs. In the case of cushioning materials of the polyurethane foam type, high-resiliency foams obtained from a polyol and a polyisocyanate, each of which has relatively lower functionality and high molecular weight, have been considered to be cushioning materials close to ideal ones. According to recent human engineering considerations, foams which can provide support of more uniform distribution for a user in a lying or seating position and can smoothly follow his movements are however considered to be good cushioning materials.
Based on the foregoing viewpoint, the present inventors proceeded with an extensive investigation with a view toward developing cushioning materials of excellent properties. As a result, it was found that a foam having only small stress variations in a range of about 25-60% in terms of strain rate and a high hysteresis loss of about 50% or higher as opposed to 20-40% of the conventional cushioning materials would be preferred as cushioning materials whose development was been desired at that time.
The properties that stress variations are small in the range of about 25-60% in terms of strain rate and the hysteresis loss is about 50% or higher are however properties inherent to semirigid foams already developed for shock-absorbing performance. It was also known that such properties could be obtained by using a polyol component, whose hydroxyl number was 100-300 or so, as a polyol component upon production of a polyurethane foam. Conventional shock-absorbing foams which were available from the use of polyols having a hydroxyl number of 100-300 however had a very large permanent compression set so that they were unable to restore their initial thickness when they were repeatedly compressed over a long time likewise cushioning materials. Such conventional shock-absorbing foams were therefore practically unusable as cushioning materials even when they had a stress-strain curve and a hysteresis loss such as those required recently for cushioning materials.